1. Field of the Invention
The present invention relates to a pressure controller, a valve having a pressure controller, and a fuel cell equipped with a valve having a pressure controller.
2. Description of the Related Art
There are various known types of valves. For example, a pressure reducing valve has a function of reducing the pressure of a primary side and keeping the pressure of a secondary side constant, and a safety valve has a function of dissipating the pressure to the exterior when a set pressure is reached.
Also, valves are mainly classified as active drive type and passive drive type valves. The active drive type valve is equipped with a sensor, such as a pressure sensor, valve driving means and a controller. On the other hand, a pressure reducing passive drive type valve automatically opens and closes utilizing the bending of a diaphragm by a differential pressure or the like.
For example, for regulating a set pressure of a passive drive type pressure reducing valve, there often has been utilized a mechanism, which is equipped with a spring that transmits a force to a valve member and in which a bend state of the spring is changed by a handle or the like. As an example of such mechanism, Japanese Patent Application Laid-open No. 2004-280232 discloses a pressure reducing valve having a pressure setting mechanism with a spring.
On the other hand, compact fuel cells are attracting attention as an energy source for mounting in a compact electric instrument.
The fuel cell is useful as a drive source for the compact electric instrument, because the energy that can be supplied per unit volume or per unit weight is several times to almost ten times that provided by the prior lithium ion secondary battery.
Particularly in a fuel cell for providing a large output, it is optimal to utilize hydrogen as the fuel.
However, since hydrogen is gaseous at the normal temperature, technology for storing hydrogen at a high density in a small fuel tank is required.
The following hydrogen storage methods are known.
A first method is to compress and store hydrogen in a state of a high-pressure gas. With the gas pressure of 200 atm. in the tank, the volume density of hydrogen becomes about 18 mg/cm3. A second method is to cool hydrogen to a low temperature and to store it as a liquid. This method is capable of a high-density storage, although there is a drawbacks in that a large amount of energy is required for liquefying hydrogen and that hydrogen may spontaneously gasify and leak.
A third method is to store hydrogen using a hydrogen storage alloy. This method has a drawback in that the fuel tank becomes heavy because the hydrogen storage alloy having a large specific gravity can absorb only about 2% by weight of hydrogen, but is effective for compaction because the absorption amount on volume basis is large.
In a polymer electrolyte fuel cell, electric power generation is conducted in the following manner. As a polymer electrolyte membrane, a cation exchange resin based on perfluorosulfonic acid is often utilized.
For such membrane, for example, DuPont's Nafion is well known. A membrane electrode assembly, which is formed by sandwiching a polymer electrolyte membrane with a pair of porous electrodes bearing a catalyst, such as platinum, namely with a fuel electrode and an oxidizer electrode, constitutes a power generating cell. By supplying the oxidizer electrode with an oxidant and the fuel electrode with a fuel in such power generating cell, protons move across the polymer electrolyte membrane to generate electric power.
The polymer electrolyte membrane generally has a thickness of about 50 to about 100 μm in order to maintain mechanical strength and in order for the fuel gas not to permeate through the membrane. Such a polymer electrolyte membrane has a strength of about 3 to about 5 kg/cm2.
Therefore, in order to prevent the membrane from being broken by differential pressure, it is preferable to limit the differential pressure between an oxidizer electrode chamber and a fuel electrode chamber in a fuel cell to 0.5 kg/cm2 or less in an ordinary state and to 1.0 kg/cm2 or less even in an abnormal state.
If the differential pressure between a fuel tank and the oxidizer electrode chamber is smaller than the above-described differential pressure, the fuel tank and the fuel electrode chamber may be directly connected without any pressure reduction.
However, if the oxidizer electrode chamber is open to the atmosphere and the fuel is filled at a higher density, it becomes necessary to reduce the pressure in the course of supplying fuel from the fuel tank to the fuel electrode chamber.
Also, the aforementioned mechanism is required for starting or terminating a power generating operation and in order to stabilize the generated electric power. Japanese Patent Application Laid-open No. 2004-31199 discloses a technology of providing a compact valve between a fuel tank and a fuel cell unit, thereby preventing the fuel cell unit from breakage due to a large differential pressure, also controlling the start and termination of the power generation and stabilizing the generated electric power.
More specifically, a diaphragm is provided at the boundary between a fuel supply path and an oxidizer supply path and is directly connected with the valve to drive the valve by a differential pressure between the fuel supply path and the oxidizer supply path without utilizing electric power, thereby providing a pressure reducing valve, which optimally controls the pressure of the fuel supplied to the fuel cell unit.
For forming a compact valve, it is possible to utilize semiconductor working technologies in addition to fine mechanical working technologies.
The semiconductor working technologies are advantageous in that they can be used on a submicron level and in that they are easily capable of being used in mass production by a batch process.
H. Jerman, J. Micromech. Microeng., 4, 210, 1994 discloses an active drive type microvalve utilizing a plurality of semiconductor substrates as materials and a semiconductor working technology. This microvalve is driven by applying a voltage to PZT.
It is possible to make a valve compact utilizing conventional mechanical working/assembling technologies, or semiconductor working technologies.
However, particularly in with respect to a passive drive type valve, although it may be possible to set the pressure at a certain value in a design stage, it is difficult after the manufacture of the valve, to control the set pressure according to the situation.
This is because, when the valve is made compact, it is difficult to provide a controlling spring or a controlling handle as in the conventional pressure reducing valve, and it is also difficult to manipulate and control such controlling members.